Rich Communication Services is a Groupe Speciale Mobile Association (GSMA) programme for providing enhanced communications features such as enhanced messaging, enriched calls with multimedia content and enhanced ‘phonebook. It is typically used in conjunction with Voice over Long Term Evolution (VoLTE). GSMA allows a mobile operator to deploy two IMS networks, one for RCS and one for VoLTE. Such a deployment has the advantage that it allows a clear separation of responsibilities within the operator. A device such as a mobile phone which is capable of using both RCS and VoLTE services can use two different IMS core systems for the two different services.
In order to be able to register with two IMS networks, User equipment (UE) must know or be able to obtain the IP address of the respective Proxy Call Session Control Functions (P-CSCF) which provide an entry to the respective IMS networks. These IP addresses are typically local IP addresses, which are valid within the network in which the P-CSCF nodes are located, but not useable from outside of the network. The IP addresses are made available to the UE either by preconfiguring the UE or by P-CSCF discovery.
P-CSCF discovery is normally used to find a single IP address for the P-CSCF before a UE performs a conventional single registration, either within a home or a visited network. To enable UE's to perform dual registration in a home network, it is typical to provide a preconfigured IP address used to identify a P-CSCF providing for access to a second IMS, such as an RCS IMS. This preconfigured IP address may be provided in the form of an Open Mobile Alliance (OMA) Device Management (DM) protocol Management Object (MO). The advantage of such a method of provision of this second IP address is that it can be used for access via other non-3GPP networks such as Wi-Fi.
Although the method of pre-configuration is completely satisfactory for use when the UE is in its home network, this IP address, being a local address, cannot be used when the UE is roaming. A second Packet Data Network (PDN) connection is therefore required, linking the roaming UE to a P-CSCF for the second IMS in its own home network. This is a cumbersome solution as separate PDN connections are needed.